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Charge-transfer excitons in strongly coupled organic semiconductors

In organic semiconductors, molecules or polymers are the building blocks of desired architectures, instead of atoms as in traditional semiconductors. This has the advantage that individual motifs can be tailored synthetically for specific electronic properties. However, electronic interactions between building blocks are often complex due to energetic disorder. Here, we address the nature of photoexcitations in chiral, near-cofacial stacks of a sexithiophene derivative that exhibits strong supramolecular pi electronic coupling. Time-resolved, temperature-dependent photoluminescence measurements on these one-dimensional lattices reveal intrinsic branching of photoexcitations to two distinct species: self-trapped excitons and dark charge-transfer excitons. We have demonstrated that in organic semiconductors, the supramolecular coupling energy dominates the nature of the primary photoexcitations. The large free-exciton bandwidth is significantly larger than attainable in the most highly organized semiconductor polymer microstructures, but the primary photoexcitations are highly localised.